The fibers of a rigid fiberboard, made by the consolidation of a water-felted mat under heat and pressure, are bound together primarily by hydrogen bonding and mechanical interlocking but also by the lignin native to the fibers. Such a fiberboard is difficult to consolidate into thin, non-planar panels without causing stretch marks and even fractures in deeply molded regions or regions adjacent thereto. In a molding press, the tension and compression forces pull and push the fibers in a rigid fiberboard apart, sometimes to the breaking point. This is a particularly significant problem with fiberboards having little or no resinous binders which would flow in response to said forces to take the place of the relatively inelastic fibers which cannot flow around the contours and angles of the die set.
The fibers of a dry felted wood fiber mat, on the other hand, are loosely bound together by a synthetic thermosetting resin and can flow along with the resin during hot pressure molding.
C. C. Heritage teaches a method for improving the surfaces and strengthening contoured parts of a molded hardboard panel in Canadian Patent No. 572,073. Either dry- or water felted wood fibers may be consolidated and molded to produce contoured hardboard, according to Heritage, by covering felted mats with an overlay of a thermoplastic or thermosetting resin in the form of a film, an impregnated fabric, or a coating.
There is a problem, however, according to the teachings of Nishibori in U.S. Pat. No. 4,610,900, when a synthetic resin is mixed with a cellulosic aggregate prior to molding. The aggregate, such as pulverized wood chips, is added to the resin to prevent the residual internal stress in the molded product which leads to warping and twisting thereof. Large amounts of the cellulosic aggregate, however, hamper the flowability of the resin and produce internal stresses in the resin product to be molded. Nishibori solves the problem by: first, heating and cooling the resin product; second, removing a skin layer of resin from the surface of the product by sanding or sandblasting; and third, cutting grooves out of the resulting exposed surface. This last operation suffers from the disadvantages of loss of the material removed to make the grooves and the expense of waste collection and removal.